Indogermanische Grammatik, ed. by Jerzy Kurylowicz. Vol. 3: Formenlehre. — Heidelberg: Carl Winter, Universitätsverlag, 1969. — (Indogermanische Bibliothek. Erste Reihe: Lehr- und Handbücher).
An overview of verbal morphology in Proto-Indo-European. For the original English manuscript see here: https://www.academia.edu/39609098/INDO-EUROPEAN_GRAMMAR_Volume_III._Morphology_by_Calvert_Watkins_English-language_version_introduced_by_Jesse_Lundquist